Interface advance control in secondary recovery program by reshaping of the interface between driving and driven fluids

ABSTRACT

The interface between driving and driven fluids in a secondary recovery operation is reshaped after a cusp has developed by injection of fluids via control wells to delay the arrival of the injected driving fluid into the vicinity of a production well.

United States Patent inventor Donald L. Hoyt Houston, Tex.

Appl. No. 786,565

Filed Dec. 24, 1968 Patented July 13, 1971 Assignee Texaco Inc.

New York, N.Y.

INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY RESHAPING OF THE INTERFACE BETWEEN nnrvmc AND maven FLUIDS 18 Claims, 12 Drawing Figs.

u.s. c1 166/245, 166/263, 166/266 1111. c1. E2 lb 43/20, E21b 43/22 Field 61 Search 166/245, 263, 268, 266

[56] References Cited UNITED STATES PATENTS 3,135,325 6/1964 Parker 166/266 36,24,813 9/1960 Lindauer 166/268 3,074,481 1/1963 Habermann 16672 3 3,109,487 11/1963 Hoyt 1 166/245 3,215,198 11/1965 Willman 166/263 Primary Examiner-Ian A. Calvert Attorneys-K. E. Kavanagh and Thomas H Whaley ABSTRACT: The interface between driving and driven fluids in a secondary recovery operation is reshaped after a cusp has developed by injection of fluids via control wells to delay the arrival of the injected driving fluid into the vicinity of a production well.

PATENIED JUL 1 319m SHEET 2 BF 2 INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY RESI'IAPING OF THE INTERFACE BETWEEN DRIVING AND DRIVEN FLUIDS FIELD OF THE INVENTION This invention relates generally to the production of hydrocarbons from underground hydrocarbon-bearing fonnations, and more particularly, to a method for increasing the efficiency of the production of hydrocarbons therefrom.

DESCRIPTION OF THE PRIOR ART In the production of hydrocarbons from permeable underground hydrocarbon-bearing formations, it is customary to drill one or more boreholes or wells into the hydrocarbonbearing formation and produce hydrocarbons, such as oil, through designated production wells, either by the natural formation pressure or by pumping the wells. Sooner or later, the flow of hydrocarbons diminishes and/or ceases, even though substantial quantities of hydrocarbons are still present in the underground formations.

Thus, secondary recovery programs are now an essential part of the overall planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. 'In general, this involves injecting an extraneous fluid, such as water or gas, into the reservoir zone to drive formation fluids including hydrocarbons toward production wells by the process frequently referred to as flooding. Usually, this flooding is accomplished by injecting through wells drilled in a geometric pattern, the most common breakthrough being the five-spot.

When the driving fluid from the injection well reaches the production wells of a five-spot pattern, the areal sweep is about 71 percent. By continuing production considerably past breakthrough, it is possible to produce much of the remaining unswept portion. It would be a great economic benefit to be able to achieve a sweep of 100 percent of the hydrocarbonbearing formation. It would be an even greater benefit to be able to achieve it at breakthrough, so that it would not be necessary to produce large quantities of injected driving fluid.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development of a cusp at the interface between the driving and the driven fluids which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, a more complete areal sweep would be possible. In the commonly assigned U.S.' Pat. No. 3,393,735, issued to A. F. Altamira et al. on July 23, 1968 for Interface Advan'ce Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbon-bearing formation by employing at least three wells, penetrating such a formation, which wells are in line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the commonly assigned U.A. Pat. No. 3,109,487, issued to Donald L. Hoyt on Nov. 5, 1963 for Petroleum Production by Secondary Recovery. In both these cited patents, there is disclosed how a production control well is positioned between the injection well and the production well and is kept on production after the injected fluid reaches it. In this manner, the cusp is pinned down at the control well remaining while the area swept out by the injection fluid before breakthrough at the outer production well is increased, there is an unwanted handling of considerable quantities of injected fluid at the control well.

Another aspect to increase the sweep is disclosed in the commonly assigned US. Pat. No. 3,393,734, issued to D. L. Hoyt et al. on July 23, i968 and involves the retardation of the development of the cusp toward a production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

SUMMARY OF INTENTION It is an overall object of the present invention to provide an improvedsecondary recovery procedure involving initially at least two production wells in line with a source of driving fluid (such as an injection well or active aquifer) for exploiting a hydrocarbon-bearing formation by reshaping the interface between a driving fluid and formation fluids, after development of a cusp, into a configuration favorable to greater sweep.

A three-well group is arranged in line so that an'end well is completed for injection and the remaining two wells are completed for production. Flooding is initiated at the end well by injection of an extraneous driving fluid, such as water, thereinto and proceeds until breakthrough of the flood front occurs at the closer of the production wells in line, at which time injection via the end well to maintain flooding is suspended, and a small volume of an extraneous fluid is injected into the formation via the closer production well, at which breakthrough occurred, to drive the cusp of the flood front therefrom and reshape the interface for the next production phase. This injected volume may be determinedas a percentage of the pore volume enclosed within the drainage radius of the closer production well. (An approximation of the drainage radius is the average of thehalf distances to the nearest wells.) Then, injection is resumed at the end well and production resumed or continued at the other production well, and there intermediate well, into which the fluid was injected, is shut in. I

The interface has now been reshaped so that instead of just one point, viz the tip of the cusp, being closest to the outer production well and thereby being accelerated to early breakthrough by the radial flow gradients, all' the points over an appreciable portion of the'interface will now be at approximately equal travel time away from the production well, this travel time being greater than would have been the time for breakthrough without injection of the extraneous fluid via the control well. In addition, the flanks of the flood front are reshaped for advancing during the period of injection of the extraneous'fluid. Thus, more of the less accessible areas are swept, and the reversal of the cusp allows more sweep before breakthrough of the driving fluid into the outer production well. Examples of the extraneous fluid include produced formation hydrocarbon fluids, butane and propane, all being miscible with the formation fluids.

Other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I discloses four units of an inverted five-spot pattern;

FIG. la is illustrative of the interface advance in the form of a cusp toward a corner production well in one quadrant of such a five-spot patternundergoing secondary recovery;

FIG. 2 discloses one unit of a nine-well diagonal pattern;

FIG. 2a, corresponding to FIG. 1a, illustrates cusp accentuation in one quadrant of a nine-well diagonal pattern unit, and FIG. 2b illustrates-the effect of continued production from a control well to retard the advance of the interface toward a corner production well, i.e., pinning the cusp, in one quadrant of a nine-well diagonal pattern undergoing secondary recovery;

FIG. 3 discloses one quadrant of a nine-well diagonal pattern,.illustrating the reshaping of the cusp at the control well between the injection well and the-corner production well.

resulting from injection of a small volume of produced fluid hydrocarbons into a control well,-with all other wells temporarily shut in;

FIGS. 4a, 4b and 4c illustrate the progress of a flood front during phases of production in accordance with this method as applied in a 13 -well pattern undergoing secondary recovery; and

FIGS. 50, b and 5c illustrate the progress of a flood front in accordance with this method as applied to a 17 -well pattern undergoing secondary recovery.

The objects of the invention are achieved by use of control wells to reshape the interface after a cusp has developed thus delaying arrival of injected driving fluid into the vicinity of an outer production well, thereby obtaining more sweep and recovery of formation hydrocarbons before breakthrough of the driving fluid into that well.

The specification and the figures of the drawings schematically disclose and illustrate the practice and the advantages of the invention with well patterns and areal sweep examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweep obtained in an ideal reservoir, although the recovery from an actual sweep of a particular field may be greater or less, depending on field parameters.

Throughout the figures of the drawings, the same symbols will be maintained as follows: P,, P, and P, represent respectively production wells at the corners, along the sides, and the interior control wells of pattern or well arrangement; and, a solid circle indicates a production well, a crossed circle indicates a closed-in well, an open circle a well site, an arrowed open circle indicates an injection well, and an arrowed solid circle, a converted injection well. The diagonals x-x in FIGS. 2, 4a and 5a represents an axis of an injection well and a pair of offset production wells.

Referring to FIG. 1, there is disclosed four units of an inverted five-spot pattern wherein the corner wells of each pattern unit are production wells, while the inner central well is used for injection.

FIG. la illustrates the growth of the cusp in one quadrant of an inverted five-spot pattern unit, wherein the secondary flooding fluid is injected into ,the central well and production is maintained at the corner wells until breakthrough, to result in a sweep of approximately 71 percent.

Referring to FIG. 2, there is disclosed a nine-well diagonal pattern, essentially the five-spot pattern with control wells positioned on the diagonals between the central injection well and the corner production wells. The control wells should be spaced at least one-half the distance between the injection well and each corner production well, with the best results obtained when such control wells are positioned between threequarters and seven-eights of the distance from the injection well toward the corner production wells. With such a pattern, the invention disclosed in the cited patent to Hoyt can be employed with success to increase the sweep area over that mentioned for the basic five-spot pattern.

FIG. 2a illustrates cusp accentuation after interface breakthrough has occurred at an interior control well, P,,

located midway between the central injection well and the corner production well, P after which this well is closed in and production initiated and maintained at the corner production well until breakthrough thereat. Production from the corner well could be concurrent with that of the interior control well.

With injection of driving fluid via the central well ofa ninewell diagonal pattern, production maintained at P, (or at P, and P until breakthrough forms the interface indicated at A, P, A, in FIG. 2a, to yield a sweep of 24 percent. If the production well P, were closed to avoid handling any of the injected driving fluid, and production was initiated (or resumed) and maintained till breakthrough at the corner well, P,, the sweep indicated by the interface at B, P, B, adds 44 percent more for a total of 68 percent.

Use of control wells as disclosed by above-cited patents to Hoyt and to Altamira et al., by continuing production from P, after breakthrough even to 100 percent injected fluid, will have the beneficial effect shown in FIG. 2!), but at the cost and difficulty of handling and disposing of large quantities of the driving fluid.

Instead, if a small volume, e.g. about 15 percent of a quadrant of a pattern unit volume of produced formation hydrocarbon fluids is injected into the formation via the control well, P,, from which production has ceased, the cusp will be driven back, and flank portions of the interface or flood front will be advanced (as indicated by the line A, -A,, in FIG. 3), by the injected bubble of hydrocarbon fluids shown in section by outline D,.

When production is initiated at the corner well, P with P, shut in, and injection resumed at the central well, the interface and bubble both move toward the production well, P,.. Now, however, all the points between a" and b" on the interface are approximately the same travel time from P and as the flood front progresses, the bubble is reproduced and the points between a-b converge into the production well, yielding at breakthrough, a sweep of 87 percent, as outlined at B, P 8,. The advantage of high sweep is thereby realized without handling injection fluid.

If satisfactory production of reservoir fluid along with injection fluid is possible in a given reservoir, (it often is not), this percentage can be increased by continued production.

The improvement in sweep can be optimized in several ways. The size of the bubble, the geometry of the wells, the

relative rate distribution, judicious use of the central injection well during the reshaping phase, can be used in any given operation to mold the interface into the best shape for the next production phase. 7

FIG. 4a discloses the basic nine-spot pattern modified by the addition of four interior control wells, which can be positioned on the diagonals of the pattern for best advantage as indicated previously. It can be visualized also as a four-unit fivespot pattern, wherein the injection wells of the inverted fivespot pattern units have been converted to production wells, and the innermost production well of the four-unit five-spot pattern has been converted into an injection well. With such a conversion, the positions of the control wells have been predetermined and may not be situated for best effect.

As illustrated in one quadrant of the pattern, the first phase of the production method requires injecting driving fluid via the central well and production initiated and maintained at the interior and side wells of the pattern until breakthrough is achieved at the four interior wells, as shown in FIG. 4a, the clear area being the sweep and the right diagonals indicating the unswept in-place fluids. Then these interior production wells are converted to injection wells for receiving a produced formation hydrocarbon fluids (bubble D,), during production is continued from the side wells P, until breakthrough thereat, as illustrated in FIG. 4b, the left diagonals in this and following figures indicating the returned hydrocarbon fluids. Then, as indicated in FIG. 4c, the four side wells are closed in, production is initiated and maintained at the comer production wells P, until the interface has progressed through the stages indicated in FIG. 4c, viz the bubble D being reproduced, the interface surface a-c-b converges and breakthrough occurs. By illustrated phases of this method when applied to the nine-spot pattern as modified by the addition of four interior wells, a sweep of approximately 94 percent follows at breakthrough (when a bubble of 28 percent of a quadrant of a pattern volume unit is used at each interior well.

FIG. 5a discloses a l7-spot pattern which is formed by drilling a single injection well in a center of a 4X4 well square. In this l7-spot pattern, there are four corner production wells, two producing side wells on each side of the 4X4 well square, and four interior control wells located on the diagonals of the pattern and positioned between the central injection well and the corner production wells.

In FIGS. 5a, 5b and 5c, there are illustrated in one quadrant of the pattern, using the same symbolism as in FIGS. 4a, 4b and 4c, the last three steps or phases of the production method as applied to the l7-spot pattern. In the first phase (not illustrated) with injection maintained at the central injection well,

production is initiated and maintained at the four interior control wells, the eight side wells and each of the corner wells until breakthrough is achieved at the control wells. In the second phase, as illustrated in FIG. 5a, while production is suspended at the corner production wells but not at the side wells, the interior control wells are converted from production to injection of produced formation hydrocarbon fluids until a bubble D FIG. 5a, of about percent ofa quadrant ofa pattern unit volume has been injected into the formation. Thereupon, the control wells, P,, are shut in, injection is resumed at the central well and production resumed at corner wells P continued at side wells P,, until breakthrough of driving fluid D occurs at the side wells, as indicated in FIG. 5b. Side wells, P,, are then closed in, and production from corner wells, P maintained until breakthrough of the driving fluid thereat. During this phase the interface surface a-b, progresses as shown by the dashed outline and converges and the injected bubble is reproduced as the interface collapses, as indicated in FIG. 5c at 1),.

In any kind of a secondary recovery operation in which one fluid is moved by another toward what is essentially a point sink, such as a production well, the radial flow gradients in the vicinity of the production well inevitably cause the interface to form a cusp pointing toward that well. The various fluid and field parameters, such as permeability distribution, viscosity ratio, well geometry, types of drives, miscibility, displacement efficiencies, etc. will cause the cusp to form earlier or later and be more or less pronounced, but it will form always, and always it will be detrimental to efficient sweep of any region being considered.

The ideal sweep situation for a reservoir would be one in which the fluid interface, as it approaches the last production well, somehow would be in such a geometrical position that all points on it would require the same time to reach the well. The

method of spreading the cusp disclosed in US. Pat. No.

3,393,734 (to Hoyt et al. is successful because it approaches this ideal.

There will be times, however, when the cusp cannot be spread out, and the presently disclosed method offers a convenient and workable alternative. All that is required is that there be at least two wells substantially in line with a source of driving fluid such as an injection well or an active aquifier. The principle involved is as follows:

The point of the cusp forms along the line of strongest gradient between the injection source and the nearest production well. If fluid is injected into the production well which is here called the control well, the system is reversed and the point of the cusp is driven back and away from the next production well in line, by the fluid injected via the control well. The interface between the control well fluid and the injection well fluid becomes concave. If the proper size of bubble is introduced, to fit the particular well geometry, the concavity may be shaped such that all or most of the points on it (between a" and b," e.g. in FIG. 3) will be in gradient fields such that their time of travel to the production well will be equal.

Resumption of injection of driving fluid and of production from the well beyond the control well will yield a very high sweep. The wider the distance between points a and b, the greater, in general, will be the sweep at breakthrough of these points into the producing well. This distance can be increased if the points of injection should straddle the axis extending through the injection and production wells, such injection points being separated by about 0.1 to 0.2 of the distance between the injection and production wells. Reservoir hydrocarbon fluids are particularly suitable here for control well injection fluid, since they will fol-low most closely model study predictions, and will be recovered without difficulty in the subsequent production phase. However, any fluid of properties (particularly viscosity and miscibility) similar to the reservoir hydrocarbon fluids can be used effectively, as, for example, the previously mentioned butane and propane.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection fluid or displacement fluid, as indicated hereinabove, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbon-bearing formation is under the influence of a water drive or gas drive, or both a water and a gas drive, and also inthe instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid.

As will be apparent to those skilled in the art, in the light of the accompanying disclosure, other changes and alterations are possible in the practice of this invention without departing from the spirit or scope thereof.

I claim:

1. A method of producing formation. fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetratingsaid formation with at least three wells, a first well, a second well and a third well,

said wells being substantially in line and the second and third.

wells being on one side of said first well with said second well closer thereto, injecting an extraneous drivingfluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said secondand'third wells, producing said formation fluids including hydrocarbons from said formation via said second well, recovering formation hydrocarbon fluids from produced formation fluids, ceasing producing said formation fluids via said second well upon breakthrough of said extraneous driving fluid thereat and then injecting some of the recovered formation hydrocarbon fluids into said formation via said second well amounting to a predetermined percentage in the amount of about 15 percent of the pore volume enclosed within the approximate drainage radius of said second well to reshape the interface between said extraneous driving fluid and said formation fluids,.

thereafter closing in said second well and resuming injecting extraneous driving fluid and producing formation fluids via the first and third wells respectively.

2. In a method as defined in claim 1, producing said formation fluids including hydrocarbons from said formation via said third well, during the injection of recovered formation hydrocarbon fluids into said formation via said second well.

3. in a method as defined in claim 1, closing in the first well upon breakthrough of said extraneous driving fluid at said second well and while injecting recovered formation hydrocarbon fluids into said formation via said second well.

4. In a method of producing formation fluids as defined in claim 1, said three wells-in line being part ofa 13-well pattern,

wherein the central well of said pattern is said first well and the remaining pattern wells are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral including said second and third wellsarranged therealong, therefrom.

and eventually producing formation fluids 5. In a method of producing formation fluids as defined in' pattern until extraneous driving fluid breakthrough occurs thereat, thereupon continuing injecting extraneous driving.

fluid into said formation via said central well andproducing said formation fluids via the corner wells of saidpatternuntil extraneous driving fluid breakthrough occurs-thereat.

7. In a method as defined in claim 6, producing formation fluids from said side wells and said corner wells being concurrent.

8. In a method as defined in claim 6, producing formation fluids from said side wells and said corner wells being in turn.

9. In a method of producing fluids as defined in claim I, said three wells in line being part of a 17 well pattern, the central well being said first well and the remaining wells being production wells located in equal numbers along the sides and on the diagonals of a quadrilateral including said second and third wells arranged therealong, and eventually producing formation fluids therefrom.

10. In a method of producing fluids as defined in claim 9, continuing injecting said extraneous driving fluid via said central well and producing simultaneously from all of the remaining wells of the pattern until said extraneous driving fluid breakthrough occurs at individual production wells on said diagonals, thereupon converting said individual production wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells in the amount of percent of the pattern unit volume and producing said formation fluids via the remaining production wells until extraneous driving fluid breakthrough occurs thereat.

II. In a method of producing fluids as defined in claim 9, continuing injecting said extraneous driving fluid into said formation via said central well and producing said formation fluids via said wells spaced on the diagonals of said pattern immediately adjacent the central injection well and continuing producing therefrom until extraneous driving fluid breakthrough occurs at such production wells, thereupon converting such wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells in the amount of 15 percent of the pattern unit volume and initiating and maintaining producing from the side wells of said pattern until extraneous driving fluid breakthrough occurs thereat, producing at the comer production wells until said extraneous driving fluid breakthrough occurs thereat.

12. In a method of producing formation fluids as defined in claim I, said three wells in line being part of a nine-well diagonalpattern, wherein the central well of said pattern is said first well and the remaining pattern wells, including said second and third wells, are production wells arranged in equal numbers on the diagonals of a quadrilateral, and eventually producing formation fluids therefrom.

13. In a method as defined in claim 12, said second well being spaced away from said first well by at least three-quarters of the distance between said first well and said third well.

14. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least an injection well and an offset production well, injecting an extraneous driving fluid comprising natural gas into said formation via said injection well to displace fluids including hydrocarbons in said formation toward said production well, producing said formation fluids including hydrocarbons from said formation via said production well until said extraneous driving fluid has reached a predetermined intermediate position therebetween, thereupon injecting into said formation an extraneous fluid miscible with said formation fluids in an amount of about 15 percent of the volume affected by said production well via an intermediate well between the injection and production wells to reshape the interface between said extraneous driving fluid and said formation fluids therebetween, and producing fluids including hydrocarbons from said formation via said offset production well while injecting said extraneous driving fluid into said formation via said injection well.

15; In a method as defined in claim 14, said intermediate well being one of a pair of wells straddling the diagonal between said injection and production wells and spaced apart from each other by a distance from 0.1 to 0.2 of. the distance between the last-mentioned wells.

16. In a method as defined in claim 14, said extraneous fluid injected via the intermediate well being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

17. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production 'wells in line. with the direction of advance of said aquifer,

producing formation fluids including hydrocarbons displaced by said aquifer until the interface between the formation fluids and said aquifer reaches a production well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid of predetermined volume to reshape said interface, and producing formation fluids including hydrocarbons from said formation via the other of said pair of production wells, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

18. In a method of producing formation fluids including hydrocarbons as defined in claim 17, said predetermined volume amounting to 15 percent of the reservoir volume within the drainage radius off said production well suffering breakthrough. 

2. In a method as defined in claim 1, producing said formation fluids including hydrocarbons from said formation via said third well, during the injection of recovered formation hydrocarbon fluids into said formation via said second well.
 3. In a method as defined in claim 1, closing in the first well upon breakthrough of said extraneous driving fluid at said second well and while injecting recovered formation hydrocarbon fluids into said formation via said second well.
 4. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a 13-well pattern, wherein the central well of said pattern is said first well and the remaining pattern wells are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral including said second and third wells arranged therealong, and eventually producing formation fluids therefrom.
 5. In a method of producing formation fluids as defined in claim 4, simultaneously initiating producing said formation fluids via all of said production wells.
 6. In a method of producing formation as defined in claim 4, producing formation fluids via said wells located on the diagonals of said pattern adjacent said injection well and continuing producing therefrom until breakthrough of said extraneous driving fluid occurs, thereupon converting said aforementioned diagonal wells into injection wells and injecting into said formation via such converted wells recovered formation hydrocarbon fluids, in the amount of 15 percent of the pattern unit volume, and producing from the side wells of said pattern until extraneous driving fluid breakthrough occurs thereat, thereupon continuing injecting extraneous driving fluid into said formation via said central well and producing said formation fluids via the corner wells of said pattern until extraneous driving fluid breakthrough occurs thereat.
 7. In a method as defined in claim 6, producing formation fluids from said side wells and said corner wells being concurrent.
 8. In a method as defined in claim 6, producing formation fluids from said side wells and said corner wells being in turn.
 9. In a method of producing fluids as defined in claim 1, said three wells in line being part of a 17 well pattern, the central well being said first well and the remaining wells being production wells located in equal numbers along the sides and on the diagonals of a quadrilateral including said second and third wells arranged therealong, and eventually producing formation fluids therefrom.
 10. In a method of producing fluids as defined in claim 9, continuing injecting said extraneous driving fluid via said central well and producing simultaneously from all of the remaining wells of the pattern until said extraneous driving fluid breakthrough occurs at individual production wells on said diagonals, thereupon converting said individual production wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells in the amount of 15 percent of the pattern unit volume and producing said formation fluids via the remaining production wells until extraneous driving fluid breakthrough occurs thereat.
 11. In a method of producing fluids as defined in claim 9, continuing injecting said extraneous driving fluid into said formation via said central well and producing said formation fluids via said wells spaced on the diagonals of said pattern immediately adjacent the central injection well and continuing producing therefrom until extraneous driving fluid breakthrough occurs at such production wells, thereupon converting such wells into injection wells and injecting recovered formation hydrocarbon fluids into said formation via the converted injection wells in the amount of 15 percent of the pattern unit volume and initiating and maintaining producing from the side wells of said pattern until extraneous driving fluid breakthrough occurs thereat, producing at the corner production wells until said extraneous driving fluid breakthrough occurs thereat.
 12. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a nine-well diagonal pattern, wherein the central well of said pattern is said first well and the remaining pattern wells, including said second and third wells, are production wells arranged in equal numbers on the diagonals of a quadrilateral, and eventually producing formation fluids therefrom.
 13. In a method as defined in claim 12, said second well being spaced away from said first well by at least three-quarters of the distance between said first well and said third well.
 14. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least an injection well and an offset production well, injecting an extraneous driving fluid comprising natural gas into said formation via said injection well to displace fluids including hydrocarbons in said formation toward said production well, producing said formation fluids including hydrocarbons from said formation via said production well until said extraneous driving fluid has reached a predetermined intermediate position therebetween, thereupon injecting into said formation an extraneous fluid miscible with said formation fluids in an amount of about 15 percent of the volume affected by said production well via an intermediate well between the injection and production wells to reshape the interface between said extraneous driving fluid and said formation fluids therebetween, and producing fluids including hydrocarbons from said formation via said offset production well while injecting said extraneous driving fluid into said formation via said injection well.
 15. In a method as defined in claim 14, said intermediate well being one of a pair of wells straddling the diagonal between said injection and production wells and spaced apart from each other by a distance from 0.1 to 0.2 of the distance between the last-mentioned wells.
 16. In a method as defined in claim 14, said extraneous fluid injected via the intermediate well being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 17. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, producing formation fluids including hydrocarbons displaced by said aquifer until the interface between the formation fluids and said aquifer reaches a production well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid of predetermined volume to reshape said interface, and producing formation fluids including hydrocarbons from said formation via the other of said pair of production wells, said extraneous fluid being selected from the grOup consisting of butane, propane and produced hydrocarbon fluids.
 18. In a method of producing formation fluids including hydrocarbons as defined in claim 17, said predetermined volume amounting to 15 percent of the reservoir volume within the drainage radius off said production well suffering breakthrough. 